Polygon has this amazing article on girls and gaming: No Girls Allowed. I'm fascinated by the shiny art and cool formatting, but there's plenty of substance to back it up. Tracey Lien traces through the history of games and how gendering is essentially a marketing ploy:

According to [Eisen Agency marketing firm president Rodger] Roeser, personal-care company Procter & Gamble is the
master of selling the same product to multiple markets because of the
way it has gendered items like shampoo, body lotion, deodorants and
shower gels. He says that the difference between something like Pantene
and Old Spice is the packaging and fragrance.

Lien discusses how early games were gender neutral, and that even when the industry started becoming male dominated, designer and game writer Lori Cole thought

"Those games weren't exactly female-targeted, but it was guys who were
making them, and they were trying to make what they could with this
technology," she says. "So I don't think it was a case of the games
being designed for guys. They were just designed by guys."

Gendering didn't really kick in until the late 80s/early 90s, when some industry studies showed that boys were playing games more than girls. This essentially created the positive feedback loop leading to games becoming a boys only club.